The Biddle CHM Series of Cable Height Meters utilize ultrasonic
sound to determine the height of up to six overhead cables and
wires. The instruments are designed to address specific measuring
requirements on cables up to 75 feet (23 m). The CHM600 (C/N 659600)
and CHM600E (C/N 659600E) are designed primarily for power cable
applications, while the CHM2000 is designed for telecommunication
applications.

The Cable Height Meters emit short bursts of sound which originate
from the cone shaped transmitter. The microprocessor in the instruments
calculates the elapsed time for the bursts of sound to be reflected
from the cable back to the instruments. The result can be displayed
in either feet and inches or meters. However, before the final
measurement can be displayed, the air temperature must be measured
because the speed of sound varies at approximately 0.2 percent
per degree Celsius.

A temperature sensor, mounted at the front of the instruments,
senses the actual air temperature. This information is sent to
the microprocessor and incorporated into the algorithm. The distance
is then displayed in meters or feet and inches depending on which
was selected.